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Objective: Aspartylglucosaminuria is an inherited, lysosomal storage disease causing progressive decline in cognitive and motor functions. The aim of this study was to evaluate volumes of subcortical gray matter structures and white matter microstructure in aspartylglucosaminuria in adolescence in a longitudinal study for the first time. Methods: A boy with aspartylglucosaminuria and his healthy twin brother were imaged twice with a 3.0 T MRI scanner at the ages of 10 and 15 years. Subcortical gray matter structure volumes were measured using an atlasbased automatic method, and diffusion tensor imaging was used to evaluate the white matter microstructure of the corpus callosum and the thalamocortical pulvinar tracts. Results: The subcortical gray matter structures were smaller at onset and diminished at follow-up in the affected twin, with the exception of the amygdala which was larger and remained the size. The largest difference in volume between the twins was found in the thalami. The total gray and white matter volumes decreased in the affected twin. In diffusion tensor imaging analysis, the fractional anisotropy was decreased at onset in the affected twin compared to the healthy brother in the evaluated tracts. The axial, radial and mean diffusivity values were increased in the affected twin. The difference between the twins increased slightly at follow-up. Interpretation: The findings suggest that volumetric measurements and diffusion tensor imaging based microstructural analysis may be useful modalities for monitoring disease progression and response to emerging treatment in aspartylglucosaminuria, but further studies with more subjects are necessary to confirm the results.